Method and apparatus for feeding a gas for epitaxial growth

ABSTRACT

A liquid raw material is heated to its boiling point or higher at a vaporizer to mix the vaporized ingredient gas and a carrier gas at a mixer at predetermined concentrations. The flow of the mixed gas is adjusted while the mixed gas is heated to over its condensing point and the temperature thereof is kept. Subsequently, the mixed gas is fed to a reactor for epitaxial growth while the mixed gas is heated to over its condensing point and the temperature thereof is kept. When the temperature of a heating medium is kept constant at the vaporizer to vaporize the liquid raw material and the feeding amount of the liquid into the vaporizer is adjusted by the pressure of the gas inside the vaporizer, the liquid surface level can be controlled to be constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus forvaporizing a liquid raw material into an ingredient gas and feeding thisingredient gas as a reacting gas into a reactor for epitaxial growth.

2. Description of the Related Art

As a reacting gas fed into this type of reactor for epitaxial growth,there is mainly used a gas obtained by vaporizing a liquid raw materialsuch as dichlorosilane (SiH₂ Cl₂), trichlorosilane (SiHCl₃) ortetrachlorosilane (SiCl₄). These raw materials except dichlorosilane areliquids at room temperature and atmosphere pressure.

Hitherto, the reacting gas as an ingredient gas has been mixed with acarrier gas to feed the mixed gas into a reactor. As illustrating in,for example, FIG. 4, the method for feeding this mixed gas includes amethod of blowing a carrier gas 3 into a liquid raw material 2 to bubblethe liquid raw material 2, thereby producing a mixed gas 4 of aningredient gas obtained by vaporizing the liquid raw material and thecarrier gas 3, and feeding the mixed gas, as it is, into a reactor forepitaxial growth 6. Other methods include a method of further mixing themixed gas with diluting hydrogen gas to obtain a mixed gas whoseingredient gas has a predetermined concentration, and feeding the mixedgas into a reactor, as not illustrated in any figures, and a method offurther injecting a dopant such as phosphorus into the mixed gas andfeeding the resultant gas into a reactor. The mixed gas is fed into thereactor in such a manner as above, so as to epitaxial-grow a siliconmono-crystal thin layer on a mono-crystal silicon substrate set in thereactor.

The above-mentioned methods of feeding the mixed gas by bubbling havethe following problems.

1) In the case of using a gas cylinder having a volume for filling up a25 kg liquid raw material and the total weight of about 50 kg,considering handling by a worker, and feeding the mixed gas into pluralreactors with the single gas cylinder, the concentration of theingredient gas included in the mixed gas is easily changed, so that therespective reaction speeds in the reactors are changed. Therefore, themixed gas can be in general fed from the single gas cylinder to only onereactor.

2) The flowing amount of the ingredient gas depends on the vaporpressure of the liquid raw material which is changed dependently on thetemperature of the liquid, the pressure in the gas cylinder, and theflowing amount of the carrier gas. As a result, controlling theconcentration of the ingredient gas is complicated.

3) When the liquid raw material is consumed so that the remaining amountthereof in the gas cylinder is decreased, the period for bringing thegas produced by bubbling in contact with the liquid raw material isshortened and the temperature of the liquid raw material is lowered bylatent heat of the liquid raw material evaporated in bubbling. Thus, theconcentration of the produced ingredient gas is reduced. As a result,the reaction speed in the reactor is also reduced.

4) Because the step for producing the ingredient gas from the liquid rawmaterial by bubbling is a sort of distillation step, a very small amountof heavy metals and high boiling point impurities included in the liquidraw material remain in the liquid. Thus, as evaporation of the liquidraw material advances, the concentration of the impurities thereinrelatively rises. As a result, the amount of the impurities in theingredient gas obtained by bubbling increases as the amount of theliquid decreases.

5) Whenever the gas cylinder is exchanged, the mouth for connecting thegas cylinder to the reactor is opened to the atmosphere. Thus, at thattime substances such as water in the atmosphere come into the gasfeeding system to deteriorate the quality of epitaxially grown thinlayer.

6) Whenever the gas cylinder is exchanged, it is necessary to make atrial for checking the reaction condition for epitaxial growth in thereactor.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and anapparatus for easily setting a desired concentration and a desiredpressure of the gas for epitaxial growth.

Another object of the present invention is to provide a method and anapparatus for feeding the gas for epitaxial growth having a constantconcentration from a single gas feeding apparatus to plural reactors.

Still another object of the present invention is to provide a method andan apparatus for feeding the gas for epitaxial growth having a highpurity continually without interruption by exchanging the gas cylinder.

In order to accomplish the above-described objects, a method for feedinga gas for epitaxial growth according to the present invention comprisesthe steps of:

heating a liquid raw material at its boiling point or higher to vaporizethe liquid raw material at a vaporizer,

adjusting the flowing amount of an ingredient gas obtained by thevaporization at the vaporizer while heating the ingredient gas to overits condensing point and keeping the temperature of the ingredient gas,and

feeding the ingredient gas the flowing amount of which is adjusted intoa reactor for epitaxial growth while heating the ingredient gas to overits condensing point and keeping the temperature of the ingredient gas.

Another method for feeding a gas for epitaxial growth according to thepresent invention also comprises the steps of:

heating a liquid raw material at its boiling point or higher to vaporizethe liquid raw material at a vaporizer,

mixing an ingredient gas obtained by the vaporization at the vaporizerwith a carrier gas so that the ingredient gas will have a predeterminedconcentration,

adjusting the flowing amount of the mixed gas of the ingredient gas andthe carrier gas while heating the mixed gas to over its condensing pointand keeping the temperature of the mixed gas, and

feeding the mixed gas the flowing amount of which is adjusted into areactor for epitaxial growth while heating the mixed gas to over itscondensing point and keeping the temperature of the ingredient gas.

Furthermore, an apparatus for feeding a gas for epitaxial growthaccording to the present invention comprises:

a vaporizer for heating a liquid raw material to its boiling point orhigher to vapor it,

a first mass flow controller for controlling a vaporized ingredient gasat the vaporizer to have a predetermined mass flow,

a second mass flow controller for controlling a carrier gas to have apredetermined mass flow,

a mixer for mixing the ingredient gas and the carrier gas controlledinto the respective predetermined mass flows,

a first flow adjusting valve for adjusting the flowing amount of themixed gas of the ingredient gas and the carrier gas,

a tube for feeding the flow-adjusted mixed gas into a reactor forepitaxial growth, and

a means for heating the ingredient gas and the mixed gas to over itscondensing point and keeping the temperature of them, at the range fromthe ingredient gas feeding tube of the vaporizer to the flow-adjustedmixed gas feeding tube for the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for feeding a gas for epitaxial gas accordingto the present invention.

FIG. 2 shows the apparatus shown in FIG. 1 more specifically.

FIG. 3 shows a vaporizer in the apparatus.

FIG. 4 shows a prior apparatus for feeding a gas for epitaxial gas.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following will describe preferred embodiments according to thepresent invention.

In a preferred embodiment of the method for feeding a gas for epitaxialgrowth according to the invention, a medium for heating the liquid rawmaterial at the vaporizer is water having a temperature of the boilingpoint of the liquid raw material or higher.

Heating the vaporizer by water having a temperature of the boiling pointof the liquid raw material or higher causes the scattering in heatingtemperature to be smaller, so as to stabilize the vapor pressure of theingredient gas specified by the heating temperature.

In a preferred embodiment of the feeding apparatus according to theinvention, a plurality of the vaporizers are in parallel located, andthe ingredient gas is forwarded from the single vaporizer selected fromthe plural vaporizers into the first mass flow controller.

By locating a plurality of the vaporizers and using them alternately, itis possible that heavy metals and high boiling point impurities remainedin the liquid raw material by vaporization for a long time are preventedfrom being diffused into the ingredient gas and polluting the gas. Thus,the ingredient gas having a high purity can be forwarded into the massflow controller continuously.

A preferred embodiment of the feeding apparatus according to theinvention has a first buffer tank for storing the ingredient gasvaporized at the vaporizer, and therein the ingredient gas forwardedfrom the first buffer tank is controlled to have a predetermined massflow with the first mass flow controller.

Fluctuation in the pressure of the ingredient gas can be absorbed byarranging the first buffer tank.

In a preferred embodiment of the feeding apparatus according to theinvention, a medium for heating the liquid raw material at the vaporizeris water having a temperature of the boiling point of the liquid orhigher, and this embodiment further has a pressure sensor for measuringthe pressure of the ingredient gas inside the vaporizer or the firstbuffer tank, a second flow adjusting valve for adjusting the feedingamount of the liquid raw material into the vaporizer, and a controllerfor controlling the second flow adjusting valve on the basis of theoutput from the pressure sensor.

The vapor pressure of the gas obtained by vaporizing the liquid rawmaterial in the vaporizer becomes a fixed value, under the conditionthat it is heated at a constant temperature. When the gas in thevaporizer is consumed, the pressure in the vaporizer drops. This dropcauses the liquid raw material stored in the vaporizer to be vaporizeduntil its vapor pressure reaches the above-mentioned fixed value, sothat the amount of the liquid decreases. Namely, when the pressuresensor detects that the gas pressure in the vaporizer drops below thefixed value, the gas in the vaporizer is fed into the first buffer tankand vaporization of the liquid raw material is advanced, so that thecontroller, judging that the liquid amount is decreased, opens thesecond flow adjusting valve to feed the liquid raw material into thevaporizer.

When the consumption of the gas in the vaporizer is stopped, the gaspressure (vapor pressure) in the vaporizer rises over the fixed value,the gas is condensed so that the gas pressure will be the fixed value.Namely, when the pressure sensor detects that the gas pressure in thevaporizer rises over the fixed value, feeding the gas from the vaporizeris stopped and the gas is condensed in the vaporizer, so that thecontroller, judging that the liquid amount tends to increase, closes thesecond flow adjusting valve to stop feeding the liquid raw material intothe vaporizer. This causes the liquid surface level in the vaporizer tobe kept constant and causes the pressure of the ingredient gas forwardedfrom the vaporizer to be controlled into a constant value.

A preferred embodiment of the feeding apparatus according to theinvention, further has a second buffer tank for storing the mixed gasmixed in the mixer, and therein the flowing amount of the mixed gas isadjusted with the first flow adjusting valve on the basis of thepressure of the mixed gas in the second buffer tank.

Fluctuation in the pressure of the mixed gas can be absorbed byarranging the second buffer tank. A constant flowing amount of the mixedgas can be fed into the reactor by adjusting the first flow adjustingvalve on the basis of the pressure in the second buffer tank.

In a preferred embodiment of the feeding apparatus according to theinvention, the tube comprises a main tube connected to the first flowadjusting valve and a loop tube for connecting the main tube to theplural reactors for epitaxial growth, and the mixed gas passing throughthe main tube can flow into the loop tube along two directions.

In the case of locating a plurality of the reactors, it is possible toreduce the pressure loss of the mixed gas in the downstream sidereactors by making the mixed gas flow into the loop tube toward the twodirections.

The following will describe the present invention more specifically,referring to the attached drawings.

As illustrated in FIGS. 1 and 2, a container 10 is connected through atube 11 to a storage tank 12. The liquid raw material used in thepresent invention includes, for example, dichlorosilane (SiH₂ Cl₂),trichlorosilane (SiHCl₃) and tetrachlorosilane (SiCl₄).

Plural feeding tanks 16 are connected through a tube 13 and switchingvalve 14 to the storage tank 12. The figures illustrate two feedingtanks, but three or more tanks may be used. Vaporizers 19 are connectedthrough respective tubes 17 and flow adjusting valves 18 to the feedingtanks 16. Thus, plural vaporizers 19 are in parallel provided to thesingle storage tank 12.

As illustrated in FIG. 3 more specifically, in this embodiment thevaporizer 19 is a multi-tube type vaporizer. As a liquid raw material20, trichlorosilane (SiHCl₃) is used. As a medium for heating thisliquid raw material, water having a constant temperature over theboiling point of the liquid is used. This temperature is decided in thelight of the total pressure of the ingredient gas pressure necessary forfeeding into reactors plus the pressure lost at the feeding systemextending from the vaporizers to the reactors, because the pressure ofthe liquid raw material and the boiling point thereof have a correlativerelationship.

The boiling point of trichlorosilane is 31.8° C. and the vapor pressurethereof is 760 mmHg at the atmosphere. In order to raise theabove-mentioned total pressure to over 760 mmHg (over the atmosphere),it is necessary to raise the boiling point of trichlorosilane andforcibly heat it in the vaporizers. When the pressure of, for example,4.2 kg/cm² (i.e., 3.2 kg/cm² G, 0.412 MPa, 3090 mmHg) is needed as thetotal pressure, with reference to the known vapor pressure table oftrichlorosilane water as the heating medium is heated to 80° C. and thentrichlorosilane is heated by the water. Similarly, when the pressure of6.8 kg/cm² (i.e., 5.8 kg/cm² G, 0.667 MPa, 5000 mmHg) is needed as thetotal pressure, water is heated to 100° C. and then trichlorosilane isheated by the water. The pressure of the vaporized gas can be changedinto a desire value by changing the temperature of water as the heatingmedium as described above.

A tube 17 is connected through a switching valve 21 and a tube 22 to thebottom of the vaporizer 19. A tube 23 connected to the other end of theswitching valve 21 is connected to a non-illustrated collecting tank.Hot water 24 as the heating medium is introduced into the vaporizer 19at its lower side portion and is discharged from the vaporizer 19 at itsupper side. To the top of the vaporizer 19 a tube 25 for forwarding thevaporized ingredient gas is connected. A liquid surface level gauge 26is provided at another side of the vaporizer 19. A pressure sensor 27for measuring the pressure of the vaporized ingredient gas is equippedwith the upper inside of the vaporizer 19. The output from the pressuresensor 27 is connected to a controller 28. The controlling output fromthe controller 28 is connected to the flow adjusting valve 18.

Turning to FIGS. 1 and 2, tubes 25 from the two vaporizers 19 areconnected through a switching valve 31 and a tube 32 to a buffer tank33. Mass flow controllers 36 are connected through a tube 34 to the tank33. The vaporizers may be integrated with buffer tanks, which is notillustrated. In this case, a switching valve is disposed at the positionwhere the two tubes connected to forwarding-openings of the two buffertanks are jointed. The pressure sensor 27 may be set at the inside ofthe buffer tank, as shown in FIG. 3.

In this embodiment, hydrogen gas is used as a carrier gas. A gas suchhelium (He) gas, besides hydrogen gas, may be used. The carrier gas isstored in a tank 37. Mass flow controllers 41 are connected through atube 38 and a heater 39 to the carrier gas tank 37. The number of themass flow controllers 36 and 41 is decided according to the controllingamount of the mass flows. In this embodiment, five mass flow controllersmay be set up, respectively. The mass flow controllers 36 and 41 areconnected through a tube 42 to a mixer 43. A buffer tank 46 is connectedthrough a tube 44 to the mixer 43. A flow adjusting valve 48 isconnected through a tube 47 to the buffer tank 46.

Plural reactors 51 for epitaxial growth are connected through a tube 49to the flow adjusting valve 48. In this embodiment, ten reactors areinstalled. The tube 49 is composed of a main tube 49a connected to theflow adjusting valve 48 and a loop tube 49b for connecting the main tube49a to the reactors 51. A distributing valve 52 is located at theposition where the main tube 49a and the loop tube 49b are jointed, sothat the mixed gas which has passed through the main tube 49a will flowinto the loop tube 49b along both directions shown by arrows in FIGS. 1and 2. Heating and temperature-retaining means 53 for heating theingredient gas and the mixed gas to over their condensing points andretaining the temperature is arranged from the ingredient gas forwardingtube 25 of the vaporizer 19 to the tube 49, as illustrated in FIG. 1 butnot illustrated in FIG. 2. Specifically, the buffer tanks 33 and 46, themass flow controller 36, the mixer 43 and the flow adjusting valve 48are covered with jackets respectively, and heating andtemperature-retaining pipes are in parallel provided to the tubes 25,32, 34, 42, 44, 47 and 49 respectively, which are not illustrated in anyfigures. Hot water having a temperature over the condensing point of theabove-mentioned gas can pass through the jackets and pipes.

In the above-mentioned feeding apparatus, a gas for epitaxial growth isfed to the reactors 51 according to the following steps.

As shown in FIG. 2, the liquid raw material (trichlorosilane) in thecontainer 10, which has been transported, is transferred through thetube 11 to the storage tank 12 and stored therein. The liquid rawmaterial in the tank 12 is distributed into the two feeding tanks 16with the switching valve 14 and stored therein. The two feeding tanks 16are used alternately.

As illustrated in FIG. 3, the flowing amount of the liquid raw materialin the feeding tank 16 is adjusted to a predetermined flowing amountwith the flow adjusting valve 18 and then is fed through the switchingvalve 21 into the vaporizer 19. The liquid surface level of the liquidraw material in the vaporizer 19 is monitored with the liquid surfacelevel gauge 26 to prevent overflow of the liquid raw material. After apredetermined amount of the liquid raw material is stored, hot waterwhose temperature is controlled to be constantly 100° C., in thisembodiment, is introduced as a heating medium into the vaporizer 19. Theliquid raw material is heated by hot water instead of electricity orsteam, so that the heating temperature can be kept more stable. Theheating permits trichlorosilane, which is the liquid raw material, to bevaporized in the vaporizer 19, so as to measure the vapor pressurethereof with the pressure sensor 27. The controller 28 can control theflow adjusting valve 18 in accordance with the output from the pressuresensor 27 to adjust the feeding amount of the liquid raw material, i.e.,trichlorosilane. Thus, the liquid surface level in the vaporizer 19 andthe gas pressure therein are controlled to be kept constant. Asdescribed above, the gas pressure of trichlorosilane is controlled to be6.8 kg/cm² (5.8 kg/cm² G, 0.667 MPa, 5000 mmHg).

The ingredient gas (trichlorosilane gas) obtained by vaporization storesin the buffer tank 33 and is kept therein. This makes it possible toabsorb fluctuations in pressure of the ingredient gas. The mass flow ofthe ingredient gas forwarded from the buffer tank 33 is controlled withthe mass flow controllers 36 to be constant. The mass flow of thecarrier gas (hydrogen gas) fed from the carrier gas tank 37 and heatedwith the heater 39 is also controlled with the mass flow controllers 41to be constant. The heater 39 allows the carrier gas to be heated to thesame temperature as that of the ingredient gas. The control by the massflow controllers 36 and 41 is carried out dependently on theconcentration of the ingredient gas necessary in the reactors. Thismakes it possible to mix the gases to have desired concentrations.

The mass flow-controlled ingredient gas (trichlorosilane) and carriergas (hydrogen gas) are mixed in the mixer 43 and kept in the buffer tank46, thereby absorbing fluctuations in pressure of the mixed gas. Apressure sensor, which is not illustrated in any figures, is set up atthe inside of the buffer tank 46 to adjust the flowing amount of themixed gas forwarded from the buffer tank 46 with the flow adjustingvalve 48 on the basis of the output from the sensor. The mixed gas whoseflow is adjusted to be constant is distributed into the loop tube 49balong its two directions with the distributing valve 52. Thedistribution along the two directions makes it possible to reduce lossin pressure of the mixed gas in the downstream side reactors of theplural ones. In other words, the mixed gas is stably fed into the pluralreactors 51 at a respective predetermined concentration, so that siliconmono-crystal thin layers are epitaxially grown at predetermined speedson the mono-crystal silicon substrates located in the reactors.

During the period when the vaporizer 19 is operated and the mixed gas isfed into the reactors 51, the buffer tanks 33 and 46, the mass flowcontrollers 36, the mixer 43, the flow adjusting valve 48, and the tubes25, 32, 34, 42, 44, 47 and 49 are heated to over the condensing point ofthe ingredient gas or the mixed gas, and the temperature are retained.This results in preventing fluctuations in the concentration of theingredient gas in the mixed gas.

After the vaporizing operation of the liquid raw material in the singlevaporizer 19 is conducted for a certain time, the liquid raw material isvaporized in another vaporizer 19. When the pressure of the vaporizedingredient gas in another vaporizer reaches a predetermined value, theswitching valve 31 is switched, thereby making it possible to preventheavy metals and high boiling point impurities remained in the liquidraw material by vaporization for a long time from polluting theingredient gas and to continuously keep the ingredient gas having a highpurity in the buffer tank 33. A used liquid raw material is dischargedby switching the switching valve 21 in FIG. 3 and forwarded to thenon-illustrated collecting tank.

In the present embodiment, the ingredient gas and the carrier gas aremixed to be fed into the reactors. In the present invention, however,only the ingredient gas, which contains no career gas, may be fed intothe reactors.

As described above, the present invention makes it possible to adjustthe flow of the liquid raw material by vaporizing it so as to have apredetermined concentration at the vaporizer, not by the prior bubblingmanner, and to set the concentration and the pressure of the gas forepitaxial growth into desired values easily. It is also possible toproduce the ingredient gas dependently on the volume of the vaporizer,thereby feeding the gas for epitaxial growth with a constantconcentration stably from the single gas feeding apparatus into theplural reactors.

Furthermore, it is possible to feed the gas for epitaxial growth havinga high purity continuously, without the prior interruption resultingfrom exchanging the gas cylinder. If, in particular, the liquid rawmaterial is vaporized at the vaporizer under the condition that thetemperature of the heating medium is kept constant and the feedingamount of the liquid raw material into the vaporizer is adjusted bycontrolling the gas pressure in the vaporizer, the liquid surface levelof the vaporizer can be controlled to be kept constant.

We claim:
 1. An apparatus for feeding a gas for epitaxial growth,comprising:a vaporizer for heating a liquid raw material to vaporize theliquid raw material by water having a temperature of the liquid rawmaterial's boiling point or higher, a first mass flow controller forcontrolling a vaporized ingredient gas at the vaporizer to have a firstpredetermined mass flow, a second mass flow controller for controlling acarrier gas to have a second predetermined mass flow, a mixer for mixingthe ingredient gas and the carrier gas controlled into the respectivefirst and second predetermined mass flows, a first flow adjusting valvefor adjusting the flowing amount of the mixed gas composed of theingredient gas and the carrier gas, a tube for feeding the flow-adjustedmixed gas into a reactor for epitaxial growth, a means for heating theingredient gas and the mixed gas to over the respective condensingpoints for the ingredient gas and the mixed gas and maintaining theingredient gas and the mixed gas at a temperature above the respectivecondensing points between the ingredient gas feeding tube of thevaporizer and the flow-adjusted mixed gas feeding tube for the reactor,a pressure sensor for measuring a pressure of the ingredient gas insidethe vaporizer, a second flow adjusting valve for adjusting the feedingamount of the liquid raw material into the vaporizer, and a controllerfor controlling the second flow adjusting valve on the basis of pressuremeasured by the pressure sensor.
 2. A feeding apparatus according toclaim 1, further comprisingat least one additional vaporizer connectedin parallel to the vaporizer, a means for forwarding the ingredient gasfrom a single vaporizer selected from the parallel-connected vaporizersinto the first mass flow controller, and a storage tank for storing theliquid raw material,wherein each additional vaporizer has a separateflow adjusting valve connected to the storage tank for adjusting thefeeding amount of the liquid raw material from the storage tank intoeach additional vaporizer.
 3. A feeding apparatus according to claim 1,which has a first buffer tank for storing the ingredient gas vaporizedat the vaporizer, and wherein the ingredient gas forwarded from thefirst buffer tank is controlled to have a predetermined mass flow withthe first mass flow controller.
 4. A feeding apparatus according toclaim 1, which further has a second buffer tank for storing the mixedgas mixed in the mixer, and wherein the flowing amount of the mixed gasis adjusted with the first flow adjusting valve on the basis of thepressure of the mixed gas in the second buffer tank.
 5. A feedingapparatus according to claim 1, further comprising at least oneadditional reactor for epitaxial growth, wherein the tube comprises amain tube connected to the first flow adjusting valve and a loop tubefor connecting the main tube to the additional reactor for epitaxialgrowth, and the mixed gas passing through the main tube can flow intothe loop tube along two directions.